The present embodiments relate to a switching device for switching a high operating voltage.
DE 101 35 835 C1 discloses a switching device for switching at a high operating voltage. The switching device combines one low voltage (LV) and at least one high voltage (HV) switching element. This type of combining is called cascade switching. The switching device is used for switching a high electric current and is also capable of safely blocking a high operating voltage. The low voltage switching element is a self-blocking metal-oxide-semiconductor field-effect transistor (MOSFET). A number of high voltage switching elements, which are serially wired with respect to each other (i.e., a cascade circuit), is realized in the form of self-conducting blocking layer field effect transistors (JFETs).
The voltage stability of the switching device is achieved by the serial wiring of the number of the high voltage switching elements. A protective element is provided between the respective control connectors of the high voltage switching element, which causes an automatic take-along effect. The protective element is switched between the two high voltage control connectors in that it has a let-pass behavior in the direction from the first to the second control connector, and in the opposite direction a blocking behavior. As soon as the first high voltage switching element is brought into a blocking state, via the externally controlled low voltage switching element, the further high voltage switching element changes into the blocking state.
If a switching device is required for a higher blocking voltage in the range of more than 50 kV, it can be switched by the switching device described above. However, the switching device represented in DE 101 35 835 C1 has the disadvantage that switching-on of the serially wired high voltage switching elements takes place in steps (intervals) by the breakdown of the serially wired protective elements. The switching device cannot be used in those applications in which the simultaneous opening of the switching elements is required.
When operating X-ray tubes in computer-controlled tomography systems, serially wired high voltage switching elements open or close simultaneously, or at least synchronously in relation to each other. A high direct current (d.c.) voltage of approximately 55 kV is generated by a high voltage network device. The radiation source is operated in a pulsed manner for reducing the radiation exposure, so that a rapid discharge of the existing capacitors must take place. The complete discharge of the (high voltage) capacitors (capacitors and lines, such as cables, for example) by the switching device should take place within 100 to 200 μs.
Switching devices for such applications include the serial circuit consisting of a number of thyristors or IGBTs (Insulated Gate Bipolar Transistors), each of which is coupled with a triggering circuit. It is problematical that these active triggering devices are connected to different electrical potentials. Accordingly, a considerable technical outlay is required, which also leads to increased costs.